TEPCO Fukushima Daini Nuclear Power Station Research on the status of response to the Tohoku-Pacific Ocean Earthquake and Tsunami and Lessons ...

 
TEPCO Fukushima Daini Nuclear Power Station Research on the status of response to the Tohoku-Pacific Ocean Earthquake and Tsunami and Lessons ...
TEPCO Fukushima Daini Nuclear Power Station

Research on the status of response to the Tohoku-Pacific Ocean

   Earthquake and Tsunami and Lessons learned therefrom

                         (Proposals)

                        October 2012

                Japan Nuclear Safety Institute
TEPCO Fukushima Daini Nuclear Power Station Research on the status of response to the Tohoku-Pacific Ocean Earthquake and Tsunami and Lessons ...
Table of contents

1.  Introduction......................................................................................................................2
2.  Overview of Fukushima Daini Nuclear Power Station .................................................4
  2.1 Overall Layout...........................................................................................................4
  2.2 System Configuration ..............................................................................................5
  2.3 Power Supply System ..............................................................................................7
  2.4 Severe Accident Countermeasures; Accident Management ................................9
3. Overview of the tsunami caused by the Tohoku-Pacific Ocean Earthquake ........... 11
  3.1 Overview of the Earthquake and the tsunami...................................................... 11
  3.2 Results of observation at Fukushima Daini .........................................................15
  3.3 Data on the earthquake and subsequent tsunami...............................................16
  3.4 Damage of Equipment............................................................................................17
    3.4.1 Damage by the earthquake .............................................................................17
    3.4.2 Damage by the tsunami ..................................................................................19
4. Response to the accident at Fukushima Daini ...........................................................23
  4.1 Response status from the time of the earthquake and tsunami until restoration
         and cold shutdown ................................................................................................29
  4.2 Emergency response situation .............................................................................30
    4.2.1 Immediately after the earthquake...................................................................30
    4.2.2 Immediately after the arrival of the tsunami..................................................33
    4.2.3 Responses toward restoration after the arrival of the tsunami...................34
       4.2.3.1   Overview of the power plant ....................................................................34
       4.2.3.2   Situation of Fukushima Daini Unit 1. ......................................................38
       4.2.3.3   Situation of Fukushima Daini Unit 2. ......................................................39
       4.2.3.4   Situation of Fukushima Daini Unit 3. ......................................................41
       4.2.3.5   Situation of Fukushima Daini Unit 4 .......................................................42
  4.3 Cooling of spent fuel pool of Fukushima Daini ...................................................44
5. Analysis of accident response.....................................................................................45
  5.1 Purpose of the analysis of accident response ....................................................45
  5.2 Concept of analysis................................................................................................45
  5.3 Specific analysis procedures ................................................................................46
  5.4 Important lessons obtained from the analysis ....................................................59
6. Emergency response as seen from the viewpoint of the human factor ..................61
7. Lessons ..........................................................................................................................64
  7.1 Organization, management, communication .......................................................64
  7.2 Advance preparation (equipment, manual, training)...........................................65
  7.3 Initial response in the accident .............................................................................66
  7.4 Additional measures ..............................................................................................67
8. Conclusion .....................................................................................................................68
Attachment Recommended Actions for the Emergency Response by
Equipments/Materials Reinforcement ................................................................................69
TEPCO Fukushima Daini Nuclear Power Station Research on the status of response to the Tohoku-Pacific Ocean Earthquake and Tsunami and Lessons ...
1. Introduction.

For everyone associated with nuclear power, March 11, 2011 has become a day which will
never be forgotten. This day will be long remembered by those in the nuclear industry for
years to come.

Even now, a large number of people still have to endure stressful lives as refugees. If we
consider the length of time as well as the size and range of the damage, we will profoundly
recognize the enormity of the damage of the nuclear disaster.

Meanwhile, TEPCO’s Fukushima Daiichi Nuclear Power Station (hereinafter referred to as
"Fukushima Daiichi"), is in the middle of a restoration process for accident convergence in
accordance with the roadmap, and the situation of the power plant may be considered to have
subsided substantially.

National agencies, local governments, industries, academia, volunteer groups and local
residents are making efforts for decontamination and other tasks toward the restoration of the
region, and as a result, some areas have managed to relax access restriction. Although there
were the most stringent controls on the shipment of food from the outset of the accident,
restrictions have been lessened little by little but it is still necessary to continue the constant
monitoring.

The level of radiation to which residents are exposed is regulated based on knowledge of the
dose rate obtained from historical data, including survivors of the atomic bombs dropped on
Hiroshima and Nagasaki, but this is not considered to provide clarity in indicating the
late-onset effects of radiation exposure, and we do not think that a situation will develop in
which there will be significant increase in cancer incidence due to radioactive materials. The
national government will continuously monitor the health of residents in the future.

Extensive validations have been performed about the Fukushima Daiichi accident, and many
reports have been published. Our Institute has also issued a proposal with the cooperation of
TEPCO and manufacturers which was published at the end of last October as a statement
from the industry. It focuses primarily on the hardware to combat tsunami to prevent accidents
from expanding. We believe that the expansion of this accident and the massive release of
radioactive materials into the environment could have been prevented if appropriate
measures had been taken.

Dr. Hatamura, Chairman of the Accident Investigation Committee of the government, stated in
a special NHK program that "this accident at Fukushima Daiichi could have been averted if
proper and adequate preparations had been in place.”

There is an ongoing national discussion of what to do about the energy supply in the future;
we think that inexpensive natural energy is not sufficient at present, and will not be secure in
the future, and that considering big problems such as energy security, global warming, and
remaining internationally competitive (the hollowing out of domestic industry), it would be
unrealistic to eliminate all nuclear power entirely.

Before the accident at Fukushima Daiichi, the Democratic Party, the current ruling party,
promised the international community a 25% reduction of CO2 emissions and was planning to
increase the proportion of nuclear power to 40%. How we can achieve our international
commitment to CO2 reduction without nuclear power? And how long we can continue to pay
as much as three trillion yen each year to operate thermal power plants instead of nuclear
power plants, in addition to buying fossil fuels? In view of these difficult issues, we think that
the realistic option for the time being is to continue using nuclear power while improving its
safety.

Following the industry report analyzing the Fukushima Daiichi accident, our Institute decided

                                                2
to validate the post-tsunami response situation at the Fukushima Daini Nuclear Power Station
(hereinafter referred to as "Fukushima Daini") which successfully led the station to
convergence, both from a technical perspective and from the viewpoint of a third party, with
the intent of gathering lessons in order to enhance the accident response capability of nuclear
power plants and contribute to the improvement of safety.

Details of the sequence of events and responses to the Fukushima Daini accident have
already been fully described in the accident analysis reports such as by the Government and
by TEPCO. Therefore, this report is dedicated to summarizing the overview of the sequence
of events and accident responses, and it is intended to pick out good practices which include
lessons for the future. In compiling these lessons, we tried to make our recommendations as
specific as possible by describing the necessary level of preparedness based on the actual
accident responses.

We hope that the lessons of this report will be utilized when nuclear power plants in Japan and
throughout the world consider possible measures against accidents.

As stated above, our report on the Fukushima Daiichi accident, which primarily summarizes
the hardware and recommends various measures, is available at the Japan Nuclear Safety
Institute’s website (former Japan Nuclear Technology Institute). Please visit the page.

In this report, we basically excluded the analysis of the activities of the central and local
governments and confined our analysis to gathering lessons about aspects to which electric
utility companies should be able to respond.

                                              3
2. Overview of Fukushima Daini Nuclear Power Station
 2.1 Overall Layout

 Fukushima Daini is located in the towns of Naraha and Tomioka in Futaba-gun, Fukushima
 Prefecture, about 12 km south of Fukushima Daiichi, facing the Pacific Ocean to the east.
 The site is approximately 147 million m2 and its shape is almost square.

 At present, four units of boiling water reactors have been installed and are arranged in the
 order of No. 1, 2, 3, and 4 from the south. The generating capacity of these units is 1,100
 MW each, making for a total installed capacity of power generation of 4,400 MW.

 When the recent disaster occurred, all units 1 to 4 were in operation at the rated thermal
 output.

 One central control room controls two units of reactors in a twin plant design: Units 1 and 2
 make up one pair and Units 3 and 4 another.

                                              Waste Treatment
                                                 Building

                                                                                    Unit 4

                                                                                    Unit 3

                         Main Office                                                Unit 2
                          Building

                                                                                    Unit 1

                             Important Seismic
                              Isolation Building                 Reactor Building
                                                                 Turbine Building
                                                                 Seawater Heat
                                                                 Exchanger Building

                    Start of                       Type                                        Situation when
Location    Unit                                                     Output(MW)
                   Operation       Reactor         Pressure Vessel                            event happened
 Naraha      1       S57.4                             Mark II           1,100
  town       2       S59.2                                               1,100               In operation at the
                                    BWR5             Mark II R                                  rated thermal
Tomioka      3       S60.6                                               1,100                      output
                                                     Advanced
  town       4       S62.8                                               1,100
                         Fig 2.1 Overall Layout of Power Stations

                                                     4
2.2 System Configuration

The system configuration of each unit of Fukushima Daini is as shown in Fig 2.2.
The role of each system is as follows:

• Reactor Core Isolation Cooling System (RCIC)
  In the event that the main condenser is no longer available for any reason, such as
  closure of the main steam isolation valve during normal operation, steam from the
  reactor will activate the turbine drive pump to inject the water in the condensate storage
  tank (referred to as "CST" below) into the reactor, and reduce the pressure by removing
  the decay heat of the fuel. The system also works as an emergency water injection
  pump in case of the failure of the water supply system, etc., and maintains the water
  level in the reactor.

• Residual Heat Removal System (RHR)
  After shutting down the reactor, the system will cool the coolant (remove the decay heat
  of the fuel) using pumps and the heat exchangers, or maintain the level of reactor water
  by injecting cooling water in case of an emergency (part of ECCS). The system has the
  ability to bring the reactor to cold shutdown and has five modes of operation: reactor
  shutdown cooling mode, low-pressure injection mode (ECCS), containment spray mode,
  pressure suppression chamber cooling mode, and emergency heat load mode.

• Emergency Core Cooling System (ECCS)
  The system consists of four subsystems: low pressure core spray system (LPCS), low
  pressure water injection system, high pressure core spray system (HPCS) and
  automatic depressurization system. In the case that a loss-of-coolant accident (LOCA)
  has occurred due to a piping break in the reactor coolant pressure boundary, such as
  the primary loop recirculation system piping, the system will remove the residual heat
  and the decay heat of the fuel in the reactor core, preventing the fuel cladding tube from
  being damaged by the fuel overheating, and consequently, minimize and suppress the
  water-zirconium reaction to a negligible extent.

• Standby Liquid Control System (SLC)
  If and when control rod insertion becomes impossible for any reason during reactor
  operation, the system will inject a neutron-absorbing boric acid solution from the bottom
  of the reactor core as a backup for the control rod to stop the nuclear reaction.

                                            5
Water Pump                         Main Turbine
                                                                                 (Turbine driven,
                     RCIC Pump                                                           2 units)
                                                                                                                       Condenser

            CST, from                                                            Water Pump      Condensate Pump
            Suppression Pool                                                    (Motor driven, (High Pressure, 3 units)
to Suppression                                                                        2 units) (Low Pressure, 3 units)
Pool
                                                                                                                           Circulating Water
                                                                                                                           Pump (3 units)
                                                                                                                                                      Sea

         from MUWC to RHR (B) Pump                                                                                       Condensate Storage
         Pump                                                                                                               Tank (CST)

                                                                                                                       to RHR
 HPCS Pump
                                                to RCIC                                                                   MUWC Pump *2
                                                                                  RHR (C)
                                                  CRD Pump,                       Pump
                                 RHR (A) Pump                                                                                     to HPCS RCIC CRD
                                                  2 units
                                                                                RHR (B)
                                                                                Pump
                                                                     from PLR (A)
                 from CST                                 from CST
  SLC Tank

                                   SLC Pump,                               LPCS Pump             *1 PLR (A) in Unit1, PLR (B) in Unit 2
                                   2 units                                                       *2 Pump three units in Unit 1, two units in Unit 2

                            Fig 2.2-1 System configuration of Fukushima Daini Unit 1 and 2

                                                                                    Water Pump                         Main Turbine
                                                                                 (Turbine driven,
                    RCIC Pump                                                            2 units)
                                                                                                                        Condenser

       CST, from CST, S/P                                                        Water Pump
                                                                                (Motor driven,      Condensate Pump
to Suppression                                                                        2 units)          (3 units)
Pool
                                                                                                                           Circulating Water
                                                                                                                           Pump (3 units)
                                                                                                                                                      Sea
          from MUWC to RHR (B) Pump
          Pump                                                                                                         Condensate Storage
                                                                                                                          Tank (CST)

                                                                                                                      to RHR
  HPCS Pump
                                                to RCIC                                                                      MUWC Pump
                                                                                  RHR (C)                                    (3 units)
                                                  CRD Pump,                       Pump
                                 RHR (A) Pump                                                                                    to HPCS RCIC CRD
                                                  2 units
                                                                                RHR (B)
                                                                                Pump
                                                                     from PLR (B)
                 from CST                                 from CST
  SLC Tank

                                   SLC Pump,                                LPCS Pump
                                   2 units

                            Fig 2.2-2 System configuration of Fukushima Daini Unit 3 and 4

                                                                       6
2.3 Power Supply System

The electricity generated by these units is transmitted via two 500 kV lines (Tomioka Line)
to the power grid. The transmission capacity of one Tomioka line is sufficient for all the
electricity generated at Fukushima Daini, and therefore the power plant can continue full
output generation even in the case of failure in one transmission line.

The power plant receives the power for starting and shutting down the reactor via two
Tomioka Lines as the main circuits, or via two 66 kV lines (Iwaido Line) as the backup
circuit.

In the event of a blackout of these two Tomioka Lines and two Iwaido Lines, the emergency
electricity to safely shut down the reactor is powered by emergency diesel generators (D/G)
and D/G in the high pressure core spray system (HPCS).

The Iwaido and Tomioka lines are shared by all units.

                                            7
500 kV            Tomioka Line                                                                                              66 kV                        Iwaido Line

            Series 1              Series 2                                                                                                Series 1 Series 2
                                                                                                                                                                                                                                       Circuit Breaker ON

                                                                                                                                                                                                                                       Circuit Breaker OFF (Standby)
                                                                                                                                                                                                                                                        Charging
                                                                                                                                                                                           66 kV Electric
   G                  G                  G              G                                                                                                                                 Boiler Switching
                                                                                                                                                                                               Station
 Unit 1       Unit 2               Unit 3 Unit 4
                                                                                                                                                                                                                              Startup Switching Station
   High Voltage
Startup Transformer

Startup Transformer                                  Startup Transformer                                                                  Startup Transformer                                                                      Startup Transformer
        1SA                                                  1SB                                                                                  3SA                                                                                      3SB

  6.9 kV M/C                6.9 kV M/C                      6.9 kV M/C                       6.9 kV M/C                                         6.9 kV M/C                                  6.9 kV M/C                              6.9 kV M/C                                  6.9 kV M/C
       1SA-1                     1SA-2                           1SB-1                            1SB-2                                              3SA-1                                       3SA-2                                   3SB-1                                       3SB-2

            Note
   Unit 1

             D/G(HPCS)                                                                      D/G(A)                                                                                                                        D/G(B)
                          D/G      *1                                                        D/G                    *3                                                                                                     D/G         *5
                                                                 LPCS Pump

                                                                             RHR Pump (A)

                                                                                                                                                                                            RHR Pump (B)

                                                                                                                                                                                                           RHR Pump (C)
              HPCS Pump

                                                                                                          EECW Pump (A)
                                                                                                                          RHRC Pump (A)
                                                                                                                                          RHRC Pump (C)

                                                                                                                                                          RHRS Pump (A)
                                                                                                                                                                          RHRS Pump (C)
                                    HPCS Pump

                                                HPCS Pump

                                                                                                                                                                                                                                        EECW Pump (B)
                                                                                                                                                                                                                                                        RHRC Pump (B)

                                                                                                                                                                                                                                                                        RHRC Pump (D)

                                                                                                                                                                                                                                                                                        RHRS Pump (B)
                                                                                                                                                                                                                                                                                                        RHRS Pump (D)

                                                                                                                                                                                                           Note:※1,※3,※5 shall be replaced to read;
                                                                                                                                                                                                                             Replace to read
                                                                                                                                                                                                                  Unit 1 Unit 2 Unit 3 Unit 4
                                                                                                                                                                                                                   *1      *2        *7       *8
                                                                                                                                                                                                                   *3      *4        *9      *10
                                                                                                                                                                                                                   *5      *6       *11      *12

                          Fig 2.3-1 Power system diagram, skeleton diagram of emergency power system

                                                                                                                           8
2.4 Severe Accident Countermeasures; Accident Management

  Table 2.4 describes Fukushima Daini’s accident management.
  Countermeasures are prepared to mitigate the effects in the event that the situation has
  expanded to a severe accident.

                    Table 2.4 Established Severe Accident Countermeasures

Function                                Severe Accident Countermeasures
                                        Recirculation Pump Trip (RPT)
Reactor shut down
                                        Alternate Rod Injection (ARI)
                                        Alternate Water Injection
                                        (Water injection into reactor core/containment vessel
Water injection to reactor core and     by make-up water pump/fire extinguishing pump )
containment vessel
                                        Automatic Depressurization of reactor core.
                                        (Additional interlock in ADS)
                                        Alternate heat removal (Utilization of D/W
                                        cooler/reactor coolant cleanup system)
                                        Restoration of damaged equipment in the residual heat
Heat removal from containment vessel    removal system
                                        (Procedure)
                                        Hardened Vent
                                        Power sharing (accommodation of 480 V power from
                                        the adjacent plant)
Support for safety functions            Restoration of damaged equipment in the emergency
                                        D/G
                                        (Procedure)

                                             9
Abbreviations
RPV    : Reactor Pressure Vessel                                                                        Diesel Driven
                                                                                                           Pump

                                                                                     FP System
RCIC   : Reactor Core Isolation Cooling
LPCI   : Low Pressure Coolant Injection
RHR    : Residual Heat Removal

                                                                    RCIR
                                                                                                       Electric Pump
                                                                                                                        Filtered Water
                                                                                                                             Tank

                                                                                     MUWC System
                                                                                                                  FP System

                                             LPCI Water
                                              Injection                                                Electric Pump
                             RPV

                                                                  RHR             MUWC
                             D/W                                                  System
                                                                                                                         Condensate
                                Pedestal Water Injection                                                                 Storage Tank

                                                                                                             MUWC System
                Suppression Pool
                                                            RHR                          Boundary of System
                                                           System

                                         Fig 2.4-1 Alternate Water Injection

                                                                                                                        Stack

                                                                              Reactor Building
                                                                               Exhaust Duct

                                                                       Standby Gas Treatment
                                                                           System (SGTS)

                                                                                                   Rupture Disk
                            D/W

                   Suppression Pool

                                                    Inert Gas System       SGTS System                            : Boundary of System
                                                      (Containment
                                                    Vessel Boundary)

                                               Fig 2.4-2 Hardened Vent

                                                             10
3. Overview of the tsunami caused by the Tohoku-Pacific Ocean Earthquake
 3.1 Overview of the Earthquake and the tsunami

  The Tohoku-Pacific Ocean Earthquake, which occurred on March 11, 2011, was the biggest
  earthquake that has ever been observed in Japan in terms of the main shock. In this
  earthquake, a maximum seismic intensity of 7 was observed in Kurihara City, Miyagi
  Prefecture.
  High tsunamis were observed on the coast of Pacific Ocean in Hokkaido, Tohoku, and the
  Kanto region.
  The focal region of this earthquake ranges from offshore Iwate Prefecture to offshore
  Ibaraki prefecture, with a size of about 500 km in length and about 200 km in width. The
  length of maximum slippage was estimated at more than 50 m.
  In this earthquake, several large slippages were observed in the offshore of the southern
  part of Sanriku near the trench, the northern part of Sanriku, and some areas off the coast
  of the Boso Peninsula near the trench, evidencing that multiple focal regions off the coast
  of central Sanriku, Miyagi, Fukushima, and Ibaraki concurrently slipped to generate a
  mega-earthquake of magnitude 9.0 (the fourth largest ever observed in the world). The
  Headquarters for Earthquake Research Promotion, a national center for the research and
  study of earthquakes and tsunamis, had been monitoring and evaluating the ground motion
  of individual regions with case records in the past, but had not assumed for the condition of
  quakes occurring in all of these areas in conjunction. The Expert Committee of the Central
  Disaster Management Council has also asserted that this enormous earthquake with a
  magnitude of 9.0 occurred with several source regions linking together in conjunction,
  which could not have been assumed from the past few hundred years of our country’s
  earthquake history.
  The tsunami that occurred due to this earthquake and caused the most extreme
  devastation along the Pacific Ocean coast in the northeastern region of Japan was a
  magnitude 9.1 on the tsunami scale, making it the fourth largest ever observed in the world
  and the largest in Japan.

                                             11
Earthquake off Sanriku, March 11, 2011 2:46 pm
                       Seismic Intensity Map

                                                                   Legend
                                                                   Seismic
                                                                   Intensity 7
                                                                   Seismic
                                                                   Intensity 6+
                                                                   Seismic
                                                                   Intensity 6-
                                                                   Seismic
                                                                   Intensity 5+
                                                                   Seismic
                                                                   Intensity 5-
                                                                   Seismic
                                                                   Intensity 4
                                                                   Seismic
                                                                   Intensity 3
                                                                   Seismic
                                                                   Intensity 2
     Epicenter                                                     Seismic
                                                                   Intensity 1

                 Source: Japan Meteorological Agency
(Earthquake off Sanriku, March 11, 2011 2:46 pm Seismic Intensity Map)

                                 12
Seismic Source
                                                                Time Function

                                                                         sec.

                                                             Starting point of bedrock destruction in
                                                             the main shock
                                                             Epicenter of M7+ shock after March 9

                                                             Epicenter of M5+ shock within 1 day
                                                             after main shock

                                                             Center of each small fault

                                                             Observation Point used in the analysis

                                                                   Slippage (m)
                                                                Contour Interval: 4 m

                       Source: Japan Meteorological Agency
            (March 2011 Earthquake and Volcanic Activity Monthly Report)

The tsunami is thought to have occurred due to the seabed almost directly above the
epicenter rising by about 3 m. The maximum height of tsunami run up above sea level was
observed at nearly 35 m in northern Miyako city. The height of the flooding in the northern
part of Miyako city was more than 25 m, and there were 58 km2 of flooded areas in Iwate,
327 km2 in Miyagi, 112 km2 in Fukushima, and 23 km2 in Ibaraki.

                                           13
Evidence of Tsunami                                                 [ Legend ]
                                                                                   Offshore Tohoku Flood
                                           Aomori                                  Height
                                          Prefecture                               Offshore Tohoku
                                                                                   Run-up Height
                      Hachinohe

                                    Miyako
                                            Iwate
                                          Prefecture

                     Rikuzentakata

                      Sendai

                                            Miyagi
                                          Prefecture

                                                                                         (Source)
                                                                                         • 2011 Tohoku-Pacific
                                                                                           Ocean Earthquake
                                                                                           Flood Height and
                                                                                           Run-up Height :
                                                                                           Preliminary Figures by
                                                                                           "Joint Investigation
                                          Fukushima                                        Group on
                                          Prefecture                                       Tohoku-Pacific Ocean
                                                                                           Earthquake and
                                                                                           Tsunami" (May 9, 2011)
                                                                                           Remarks: Based on the
                                                                                           Data with Reliability
                                                                                           Level A (Highly reliable
                                           Ibaraki                                         with clear trace and
                                                                                           minimal measurement
                                          Prefecture                                       error) obtained within an
                                                                                           area of 200 m from the
                                                                                           shoreline.

          Flood Height and Run-up Height
                                              Embankment
                          Flood Height

                  Run-up Height                                                         Sea level at the time of
                                                                                        tsunami arrival
                                                                                        Tokyo Peil: Mean sea
                                                                                        level at Tokyo Bay (T.P.)
          Flood Height:       Height from the sea level at the time of arrival of the
                              tsunami to the traces of tsunami inundation
          Run-up Height:      Height from the sea level at the time of arrival of the
                              tsunami to the traces of tsunami run-up.

Source: Excerpt from the 1st Meeting Material by The Investigation Committee on the
Countermeasures learned as Lessons from the Tohoku-Pacific Ocean Earthquake
As of July 31, 2012, the damage caused by the earthquake and the tsunami is enormous:
15,867 dead, 2,903 missing, 130,445 buildings totally destroyed, and 264,110 buildings
partially destroyed.

                                                          14
3.2 Results of observation at Fukushima Daini

The observed values of ground motion at the foundation of the reactor building (the lowest
basement floor) of Fukushima Daini was below the maximum acceleration of design
earthquake ground motion Ss (the maximum acceleration observed was 305 Gal on the B2
floor of the Unit 1 reactor building), and therefore this ground motion can be considered to
be within the expected range of the seismic safety evaluation of the equipment.
Furthermore, using the recorded values of ground motion at the free field obtained during
the earthquake, a soil structure model was identified for the strip analysis. As a result,
ground motion of the strip analysis proved to be generally at the same level as actual
observations, although the calculated values exceeded the design earthquake ground
motion Ss in some of the periodic bands.

                                                                                                               Recorded Measurement
                                                                                                               Basic Seismic Movement Ss-1
 Acceleration (Gal)

                                                                                  Acceleration (Gal)
                                                                                                               Basic Seismic Movement Ss-2
                                                                                                               Basic Seismic Movement Ss-3

                                              Time (sec.)

                       Fig. 3.2-1-1 Time History of Acceleration on the Ground                                     Cycle (sec.)
                           Level of Reactor Building, Unit 1 (NS Direction)                            Fig. 3.2-2-1 Response Spectrum
                                                                                                       on the Ground Level of Reactor
                                                                                                        Building, Unit 1 (NS Direction)

                                                                                                               Recorded Measurement
                                                                                                               Basic Seismic Movement Ss-1
                                                                                 Acceleration (Gal)
 Acceleration (Gal)

                                                                                                               Basic Seismic Movement Ss-2
                                                                                                               Basic Seismic Movement Ss-3

                                              Time (sec.)

                       Fig. 3.2-1-2 Time History of Acceleration on the Ground                                     Cycle (sec.)
                            Level of Reactor Building, Unit 2 (NS Direction)                           Fig. 3.2-2-2 Response Spectrum
                                                                                                       on the Ground Level of Reactor
                                                                                                        Building, Unit 2 (NS Direction)

                                                                                                              Recorded Measurement
 Acceleration (Gal)

                                                                                                              Basic Seismic Movement Ss-1
                                                                                 Acceleration (Gal)

                                                                                                              Basic Seismic Movement Ss-2
                                                                                                              Basic Seismic Movement Ss-3

                                              Time (sec.)

                       Fig. 3.2-1-3 Time History of Acceleration on the Ground                                    Cycle (sec.)
                            Level of Reactor Building, Unit 3 (NS Direction)                           Fig. 3.2-2-3 Response Spectrum
                                                                                                       on the Ground Level of Reactor
                                                                                                        Building, Unit 3 (NS Direction)

                                                                                                              Recorded Measurement
                                                                                                              Basic Seismic Movement Ss-1
 Acceleration (Gal)

                                                                                 Acceleration (Gal)

                                                                                                              Basic Seismic Movement Ss-2
                                                                                                              Basic Seismic Movement Ss-3

                                             Time (sec.)

                      Fig. 3.2-1-4 Time History of Acceleration on the Ground                                       Cycle (sec.)
                           Level of Reactor Building, Unit 4 (NS Direction)                            Fig. 3.2-2-4 Response Spectrum
                                                                                                       on the Ground Level of Reactor
                                                                                                        Building, Unit 4 (NS Direction)

                                   Exemplification of larger horizontal direction in table

                                                                     15
3.3 Data on the earthquake and subsequent tsunami
  (1) Date and time of occurrence
    March 11, 2011, 2:46 pm

  (2) Epicenter
     Off the coast of Sanriku
     (38.1 N/142.9 E, 130 km ESE of Oshika Peninsula, focal depth 24 km)

  (3) Magnitude
    9.0

  (4) Peak ground acceleration
    305 Gal at B2 floor of the Unit 1 reactor building (Vertical)

  (5) Distance from Fukushima Daini
    183km to epicenter, 185 km to hypocenter

  (6) Data on tsunami
   a    Inundation height
    (a) Seaside area (Ground height at base level of Onahama Port Construction site
        (hereinafter O.P.) + 4 m)
     •    About +7 m*1 (flood depth about 3 m)
       *1) South side of Unit 1 seawater heat exchanger building. Highest point
   (b) Main building*2 area (Ground height O.P. + 12 m)*2 Reactor building and Turbine
       building
    •    O.P. about +12~+14.5 m*2 (flood depth less than 2.5 m)
       *2) Between the area south of Unit 1 Main building and the important seismic isolation
           building.
           Locally O.P. about+15~+16 m (flood depth about 3-4 m)
   b    Flooded area
       The flooded area extended across the entire seaside area, but no entry of tsunami
    seawater into the main building area beyond the slope from the seaside area was
    observed. The tsunami run up was mostly from the southeast side of the main building
    area toward the road leading to the important seismic isolation building. As a result,
    flooding was limited only to the area surrounding the Unit 1 and 2 buildings and the
    south side of the Unit 3 building (no flooding around the Unit 4 building).

  (7) Arrival of the first wave of the tsunami
    March 11, 2011 3:22 pm Visual sighting)

                                               16
3.4 Damage of Equipment
3.4.1 Damage by the earthquake
  The external power supply of Fukushima Daini consists of a total of four lines: 500 kV of
  Tomioka 1L and 2L and 66kV of Iwaido 1L and 2L, all from the Shin Fukushima
  substation. On the day of the earthquake, however, Iwaido 1L was out of service for
  inspection and only three lines were available.
  After the earthquake, Tomioka 2L went out of service at around 2:48 pm on March 11 due
  to damage to the breakers at the Shin Fukushima substation. After the earthquake,
  patrol inspection of facilities found damage at the arrester of Iwaido 2L, so after
  confirming that Tomioka 1L was continuously receiving all the power necessary for the
  station, Iwaido 2L was put out of service for restoration works in order to prevent the
  damage from spreading further.
  As a result, after the earthquake, the external power supply was only available from a
  single line for the time being. However, Iwaido 2L resumed service at 1:38 pm on the
  next day, March 12, and Iwaido 1L was also revived at 5:15 am on March 13 after
  temporary restoration, giving the receiving configuration three lines.
  Maintaining the external power through this single transmission line made it possible to
  supply power to the available facilities, contributing a great deal to preventing the spread
  of the accident, and led to early convergence. All the equipment such as circuit breakers,
  etc. at the substation and switching stations has basically the same specifications as at
  Fukushima Daiichi.

                                            17
Shin Fukushima Transformer Substation

                                      Under
                                   construction

                                                                                                                                                  Suspension of Electric
                                                                                                                                                  Cable (Out of Service)
Tomioka Line 1L

                                               Tomioka Line 2L

                                                                                                                           Iwaido Line Line 2L
                                                                                                     Iwaido Line Line 1L
                                                                      Tomioka Line 2L,
                                                                         Suspended
                                                                    (Suspended after the
                                                                       earthquake but
                                                                     before the tsunami)
                                                                             Iwaido Line Series 1,                                                 Switching Station
                                                                          Suspended for repair works                                              for Auxiliary Boiler

                                                                             Switching                                                    66 kV Startup Switching Station
                                                                             Station for
                                                                            Transformer
                  Unit 1       Unit 2      Unit 3                Unit 4       Start-up

                           Ordinary High Voltage Power Panel                               Ordinary High Voltage Power Panel
                                         (M/C)                                                           (M/C)

                    Emergency High                   Emergency High                     Emergency High                                             Emergency High
                     Voltage Power                    Voltage Power                      Voltage Power                                              Voltage Power
                      Panel (M/C)                      Panel (M/C)                        Panel (M/C)                                                Panel (M/C)

                                         Fukushima Daini Nuclear Power Station
                                                                                                                                                 Out of service for check and maintenance
                                                                                                                                                 Trip (Suspended)
                                                                                                                                                 Circuit Breaker
                                                                                                                                                 Dis-connector
                                                                                                                                                 Transformer
                                                                                                                                                 Main Power Generator

                    Fig 3.4.1-1 Fukushima Daini Outline Diagram of External Power Supply

                                                                                   18
3.4.2 Damage by the tsunami
 (1)      Entry of flood water into Main building (Ref: Fig 3.4.2-1, 3.4.2-2)
   Inundation surrounding the main buildings of Fukushima Daini (reactor building and
   turbine building; Ground Level OP +12 m) was not significantly deep, with the
   exception of intensive run up on the south side of Unit 1.
   Intensive tsunami run up entered the Unit 1 building through the openings at ground
   level located on the south side of the reactor building (air intake louver for the
   emergency D/G, equipment hatch on the ground level) which flooded the reactor
   building (annex building) and caused the loss of function of all three units of
   emergency D/G, emergency power supply (for the C system and high pressure core
   spray system).
   The depth of run up around Unit 2 through Unit 4 was not significant, and thus flooding
   into the reactor buildings or turbine buildings through the openings at ground level was
   not detected. However, flooding was confirmed in the basement of the reactor building
   of Unit 3 (Annex) and the basements of turbine buildings Unit 1 through Unit 3. It is
   thought that tsunami inundation entered the buildings through the cables and pipes
   leading to the underground trenches and ducts.

                   Seawater Heat
                 Exchanger Building

                                                  Unit 4             Unit 3             Unit 2         Unit 1
                  Turbine Building

                  Reactor Building

                    Openings on the ground where the flood entered
                    the main building
                    Openings leading to the underground trench
                    duct where the flood entered the main building

                    Fig 3.4.2-1 Openings of flood entrance into the main building
                                                                                                       Unit 1, Water entry through
                                                         Unit 2-4, No significant water entry          louver/hatch to the reactor
                                                         through the louver/hatch to the reactor       building annex
                                                         building annex                                           Main Reactor
                                                                                                       Reactor      Building
                                                                                                                                 Reactor
                                                                         Main Reactor                  Building                  Building
                                                                                                        Annex
                                                                           Building                                               Annex

                                     Equipment Hatch     Reactor              D/G Air    Reactor
                                                         Building             Intake Building Annex
                     Seawater Heat        Flood Height: Annex
                     Exchanger Building   O.P. + about 7 m                    Louver
         Ground Level Building                                        Equipment Hatch
                                               Ground Level
         O.P. + 4m     Entrance
                                               O.P. + 12 m

                       Power Panel                                                       Power Panel

       Seawall      Drainage Pump

           Fig 3.4.2-2 Flow path of flood to the main buildings of Fukushima Daini

                                                                    19
Table 3.4.2-1 Location of D/G and damages thereto
                             Unit 1              Unit 2                Unit 3                Unit 4
                 *1
Height of tsunami                                         About +9 m
Ground Level                                              O.P.+12 m
Flood depth around
                                  Less than 2.5 m (nearly zero except around Unit 1)
  main buildings
                                            [O.P .about+12 m~+14.5 m] *2
[Inundation Height]
                         Reactor Bldg.       Reactor Bldg.        Reactor Bldg.         Reactor Bldg.
                 A          Annex               Annex                Annex                 Annex
                            [B2F]               [B2F]                [B2F]                 [B2F]
Building
                         Reactor Bldg.       Reactor Bldg.        Reactor Bldg.         Reactor Bldg.
where D/G
is installed     B          Annex               Annex                Annex                 Annex
[Floor]                     [B2F]               [B2F]                [B2F]                 [B2F]
                         Reactor Bldg.       Reactor Bldg.        Reactor Bldg.         Reactor Bldg.
                 H          Annex               Annex                Annex                 Annex
                            [B2F]               [B2F]                [B2F]                 [B2F]

□:D/G unit flooded           *1   Estimate at tide station. Actual measurement unknown because of
                                  instrument damage.
□:D/G unit not flooded
                             *2   Locally about OP+15 – 16 m [flood depth about 3 – 4 m] in the area
                                  between the south side of the Unit 1 main building and the important
                                  quake proof building.

     (2) Damage situation
        Among the equipment for reactor cooling that was also damaged by the tsunami, the
        damage situation of the facilities that clearly show characteristics of equipment
        damage caused by the recent tsunami is explained as below.(Please refer to Table
        3.4.2-2.)

  ① Emergency equipment cooling pump
  Units 1 through 4 use seawater to remove decay heat from the reactor. The emergency
  D/G unit also uses seawater to cool its engine. Therefore, the pumps for the emergency
  equipment cooling water system (the seawater intake pump and the fresh water cooling
  pump sourced from seawater) are installed in the seaside area. These pumps are installed
  in the seawater heat exchanger building. The purpose of the overall layout in which
  seawater is not sent directly to the reactor building but to the intermediate seawater heat
  exchanger building, which is not a radiation control area, where a freshwater loop is
  installed with a heat exchanger and cooling pump for auxiliary equipment constituting an
  independent set of equipment cooling facilities, is to prevent seawater from mixing with
  reactor water and to improve maintenance works. The pumps for the emergency
  equipment cooling water system are specified for outdoor use, but they were installed
  indoors in the heat exchanger building as part of the independent set of equipment cooling
  facilities. The ground level of the seaside area where the pumps for the emergency
  equipment cooling water system are installed had OP +4 m elevation, and its precautionary
  measures ensured safety against a tsunami height of 5.1 - 5.2 m, based on the tsunami
  height evaluation results in 2002 provided by the Japan Society of Civil Engineers
  “Tsunami Evaluation Technology”. However, this tsunami greatly exceeded expectations,
  and the motors of the pumps were submerged under the water and the system lost
  function. The pumps for the emergency equipment cooling water system at Fukushima
  Daini were housed in the seawater heat exchanger buildings. The area surrounding the
  buildings was inundated to a depth of 3 m by the tsunami, and while there was no damage
  to the building structures, all the seawater heat exchanger buildings were flooded by
  seawater through the damaged doors and other openings on ground level. As a result, the

                                                20
power panels and the motor of the pumps were submerged underwater and the total of
               eight emergency equipment cooling water systems lost function except for one system of
               Unit 3. In addition, the cooling system of the three D/G units, A, B and H installed for each
               reactor Unit also lost all function except for three systems: B and H of Unit 3 and H of Unit
               4.

          ② Power panel
          The scale of the tsunami at Fukushima Daini was different from that at Fukushima Daiichi,
          and inundation of the main building was also different. Accordingly, the situations of power
          panel damage also differ as a result.
                  The Reactor building of Unit 1 (Annex), which was flooded by the tsunami, saw inundation
                  on the emergency power panels of the C and H systems, but the D system power panel
                  was not flooded and all the power panels of other reactor Units were free of damage. Due
                  to this situation, it was possible to distribute electric power received from outside sources
                  to various pieces of equipment through these emergency circuits, making it possible to use
                  the necessary facilities to cope with the emergency situation (distribution through Normal A
                  and B2, Emergency C and D2, and high pressure core spray system H). On the other
                  hand, the power panels in the seawater heat exchanger buildings located in the seaside
                  area were all submerged in the tsunami because of flooding in the buildings, and seven
                  power panels lost function, with the exception of one low voltage power panel (P/C)
                  situated in the seawater heat exchanger building of Unit 3.
                  Therefore, the residual heat removal seawater system lost function except for one system
                  of Unit 3 out of the total of eight.

                          Table 3.4.2-2 Damage situation of the cooling pumps for emergency equipment and
                          emergency P/C located in the seawater heat exchanger buildings after the tsunami
                                                                                      ○ Usable △ Out of service due to flooding of Power Panel
        Seawater Pump (RHRC, RHRS and LLCW)                                            × Out of service due to flooding of Power Panel and Motor
 Location
 Installati

                       Unit 1                   Unit 2                               Unit 3                           Unit 4
    on
    of

             North side     South side North side     South side            North side    South side         North side     South side
     Floor
     2nd   Ground Floor

                                                                                                                                      ○ Usable
        Emergency P/C                                                                                            × Out of service due to flooding
Location Grou
Installati

                       Unit 1                            Unit 2                      Unit 3                            Unit 4
   on
   of

             North side     South side          North side     South side   North side     South side         North side     South side
           Floor
            nd

                                                                       21
③ Emergency Diesel Generator
  In the Fukushima Daini power station, each reactor Unit has three (A, B, H) emergency
  diesel generators (hereinafter referred to as "emergency D/G"). Unit 1 lost all three
  emergency D/Gs because the tsunami had entered the reactor building (Annex) through the
  opening at the ground level. Some of the emergency D/Gs which were able to avoid the
  flooding still lost function because of the loss of diesel engine cooling due to the flooding of
  the power panel and the pump motor for the cooling system. The cooling system of the
  emergency D/G was lost for most of the systems except for three: B and H of Unit 3 and H
  of Unit 4. As a result, nine D/Gs lost function: A, B and H of Unit 1, A, B and H of Unit 2, A of
  Unit 3 as well as A and B of Unit 4.
  However, Fukushima Daini had a continuous supply of electric power from the external
  sources and there was no need to activate such surviving emergency D/G after all.

  ④ Situation of other outdoor damage
  In the Fukushima Daini power station area, no major equipment and/or structure was
  observed being washed up by the tsunami to the main building area (elevation OP +12 m).
  However, five cases of openings/holes caused by the tsunami washing away or damaging
  the lid of the hatch duct in the main building area have been reported.

Source;
  Tokyo Electric Power Company
     The impact of Tohoku-Chihou Taiheiyo-Oki Earthquake to Nuclear Reactor Facilities at
     Fukushima Daini Nuclear Power Station (May 9, 2012)
     Fukushima Nuclear Accident Analysis Report (June 20, 2012)

  Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo
  Electric Power Company
     Final Report (July 23, 2012)

                                                22
4. Response to the accident at Fukushima Daini

The status of the accident at Fukushima Daini has already been reported in detail in the
reports of TEPCO and the Investigation Committee of the Government. Therefore, this report
puts the emphasis on describing the situation, including points of suggestion in considering
the lessons learned, and giving a brief overview of the other points.
Fig 4-1 shows the operational status of the responses of Units 1 through 4, and Figs 4-2 to 4-5
show the operational status of each Unit in conjunction with the changes in the main
parameters of each Unit.

                                              23
March 11                                                March 12                                                                            March 13                                                                         March 14                                                    March 15                                                   March 16
         12:00    16:00 20:00              0:00          4:00         8:00   12:00                16:00         20:00         0:00          4:00          8:00   12:00               16:00         20:00           0:00       4:00         8:00   12:00          16:00    20:00     0:00      4:00     8:00   12:00        16:00     20:00    0:00      4:00      8:00   12:00       16:00     20:00    0:00
             ▽15:33 CWP(C) Manual stop                                                                                                                                                            ▲20:17 RHRS(B) Manual start
             ▲▽15:34 Emergency D/G A, B and H automatic start followed by immediate trip                                                                                                           ▲21:03 RHRC(D) Manual start
             ▲15:36 MSIV manual full-close                                                                                                                                                                 ▲1:24 RHR(B) Start  Call-off pursuant to Article 10 of the Act on Special Measures Concerning Special Law for Nuclear Emergency
              Water injection by RCIC (15:36 – 4:58)                                                                                                                                                          ▲3:39 S/C spray by RHR (B) start
                                                                                                                                                                                                                            ▲10:05 Reactor water injection by means of RHR (B) LPCI mode
                 ▽15:57 CWP(A,B) Automatic stop
                 ▲15:50~as              ▲Rapid depressurization (3:50 – 4:56)]                                                                                                                                       ▲1:44 Emergency component cooling system (B) manual start
                                                                                                                                                                                                                                                ▲16:30 SFP water injection by FPMUW start
                 appropriate      ▲0:00 MUWC alternative water injection start
Unit 1           Reactor                                                                                                                                                                                                                                 ▲20:26 FPC (B) circulation operation start
                                                 S/C cooling by FCS cooling water (MUWC)(6:20~7:20)
                 depressurization              ▲D/W spray (7:10 – as appropriate)
                                                ▲S/C spray (7:37 – as appropriate)                                                                                                                                                              ▽9:40 MUWC alternative water injection Stopped                                                ▲0:42RHR(B) SFP cooling start
                      ▲17:53 D/W cooling system manual start                                                Configuration of pressure proof vent-line (10:21 – 18:30)                                                                                            ▲17:00 Reactor cold shutdown                                                                         ▲10:30 SFP water temperature
                        Report pursuant to Article 10 of the Act on Special Measures Concerning Special Law for Nuclear Emergency (loss of reactor heat removal function) (18:33 on March 11 – 1:24 on March 14)
                                                                                                                                                                                                                                                                                                                                                                      approximately 38°C (before
                                                                                                                                                                                                                                                                                                                                                                      earthquake)
                                                              Report pursuant to Article 15 of the Act on Special Measures Concerning Special Law for Nuclear Emergency (loss of pressure suppression function) (5:22 on March 12 – 10:15 on March 14)

             ▲▽15:34 Emergency D/G H automatic start followed by immediate trip                                                                                                                                           ▲3:20 Emergency component cooling system (B) manual start
             ▲15:34 MSIV manual full-close
             ▲15:35 RHR(B) Manual start                                                                                                                                                                                     ▲3:51 RHRS(B) Manual start
             ▽15:35 CWP(C) Manual stop, (A and B) automatic stop                                                                                                                                                                ▲5:52 RHRC(B) Manual start
             ▽15:38 RHR(B) Stop                                                                                                                                                                                                   ▲7:13 RHR(B) Start (S/C cooling mode)  Call-off pursuant to Article 10 of the Act on Special Measures Concerning Nuclear Emergency
              ▲▽15:41 Emergency D/G A and B automatic start followed by immediate trip                                                                                                                                             ▲7:50 RHR(B)S/C Spray start
                  Water injection by RCIC (15:41 – 4:53)                                                                                                                                                                                ▲10:48 Reactor water injection by means of RHR (B) LPCI mode
                           ▲20:02 D/W cooling system     Configuration of pressure proof vent-line (10:33 – 10:58)
Unit 2
                           manual start
                                            ▲4:50 MUWC alternative water injection start                                                                                                                                                                            ▲18:00 Reactor cold shutdown                                                ▲1:28 RHR(B) SFP cooling start
                                                 ▲D/W spray (7:11 – as appropriate)
                                                                                                                                                                                                                                                                                                                                                                      ▲
                                                  ▲S/C spray (7:35 – as appropriate)                                                                                                                                                             ▽10:12 MUWC alternative water injection stopped                                                                      10:30 SFP water temperature
                                                                       S/C cooling by FCS cooling water (MUWP) (6:20 – 7:52)                                                                                                                                                                                                                                          approximately 32.5°C (before
                        Report pursuant to Article 10 of the Act on Special Measures Concerning special Law Nuclear Emergency Preparedness (loss of reactor heat removal function) (18:33 on March 11 – 7:13 on March 14)                                                                                                                                             earthquake)

                                                              Report pursuant to Article 15 of the Act on Special Measures Concerning Special Law for Nuclear Emergency (loss of pressure suppression function) (5:32 on March 12 – 15:52 on March 14)

             ▽15:34 CWP(C) Manual stop
             ▲▽15:35 Emergency D/G A, B and H automatic start followed by immediate trip of A
             ▲15:36 RHR(B) S/C cooling start    ▲9:37 RHR(B) Manual start (commencement of SHC)
                     ▲19:46 RHR(B) S/C cooling  Switch to LPCI mode
                      ▲20:07 RHR(B) LPCI mode  Switch to S/C cooling
                             ▲0:06 RHR(B) Commencement of configuration of SHC mode system
                               ▽1:23 RHR(B) Manual stop, prepare for SHC mode being on
                                  ▲2:39 RHR(B) Manual start (S/C cooling start)
Unit 3                            ▲2:41 RHR(B)S/C Spray start
                                             ▽7:59 RHR(B) Manual stop
                 ▲15:37 MSIV manual full-close
                 ▽15:38 CWP(B) Manual stop                                            Configuration of pressure proof vent-line (12:08 – 12:13)
                  Water injection by RCIC (16:06 – 23:58)                                                                                                                                                                                                                                                                     ▲17:42 FPC Hx cooling water switch-over (RCW  RHRC)
                                                                                   ▲12:15 Reactor cold shutdown
                   ▽16:48 CWP(A) Manual stop                                                                                                                                                                                                                                                                                                                                                ▲
                         ▲20:12 D/W cooling system manual start                                                                                                                                                                                                                                                                                       22:30 SFP water temperature approximately
                                     ▲22:53 MUWC alternative water injection start (unknown when stopped)                                                                                                                                                                                                                                                           32.5°C (before earthquake)

             ▽15:33 CWP(C) Manual stop                                                                                                                                                                                                              ▲11:00 Emergency component cooling system (B) manual start
             ▲▽15:34 Emergency D/G A, B and H automatic start followed by immediate trip of A and B
             ▽15:34 CWP(A,B) Automatic stop       ▲11:17 HPCS pump start (S/C agitation operation)                                                                                                                                                       ▲13:07 RHRS(D) Manual start
             ▲15:36 MSIV manual full-close                                                                                                                                                                                                                 ▲14:56 RHRC(B) Manual start
                                                     ▲12:32 Reactor water injection switch-over (MUWC  HPCS)                                                                                                                                                ▲15:42 Start (S/C cooling mode)  Call-off pursuant to Article 10 of the Act on Special Measures Concerning Special Law for Nuclear Emergency
             ▲15:36 RHR(A,B) Manual start               ▽13:48 Water injection by HPCS stop
             ▽15:38 RHR(A) Manual stop                                                                                                                                                                                                                        ▲16:02 RHR(B)S/C Spray start
             ▽15:41 RHR(B) Automatic stop                                                                                                                                                                                                                         ▲18:58 Reactor water injection by means of RHR (B) LPCI mode start
                  Water injection by RCIC (15:54 – 0:16)
Unit 4                  ▲19:14 D/W cooling system manual start
                                    ▲0:16 MUWC alternative water         ▽17:25 MUWC alternative water injection stopped                                                                                                                                                                             ▲7:15 Reactor cold shutdown
                                    injection start ▲7:35 Alternative containment vessel spray start
                                                                                                                                                                                                                                                                                                                           ▲16:35 FPC Hx cooling water switch-over (RCW → RHRC)
                                                                     S/C cooling by FCS cooling water (MUWP)(7:23 – 22:14)
                                                                                                                                                                                                                                                                                                                                                                                      ▲
                                                                               Configuration of pressure proof vent-line (11:44 – 11:52)
                                                                                                                                                                                                                                                                                                                                                           20:59 RHR(B) SFP cooling start
                        Report pursuant to Article 10 of the Act on Special Measures Concerning Special Law for Nuclear Emergency (loss of reactor heat removal function) (18:33 on March 11 – 15:42 on March 14)
                                                                                                                                                                                                                                                                                                                            (7:30 on March 18: SFP water temperature approximately 35°C
                                                                 Report pursuant to Article 15 of the Act on Special Measures Concerning Special Law for Nuclear Emergencys (loss of pressure suppression function) (6:07 on March 12 – 7:15 on March 15)                                                                                                            (before earthquake))

                                  ▲Around 22:00 Commence site observation (around Hx / B)                                               ▲Around 6:00 Equipment and materials arrived
                                                                                                                                                                                  ▲Around 23:30 Completion of temporary cable-laying
                                                                                                                               ▲Before dawn: Cable-laying, Unit 1 being of priority (Unit 2  Unit 1)
                                                                                                                                                  Chart 4-1: Situation of action in response to the Unit 1 – Unit 4 accident
                                                                                                                                                                                                                                  24
March 11                                              March 12                                                                            March 13                                                                         March 14                                               March 15                                           March 16
         12:00    16:00 20:00            0:00          4:00         8:00   12:00                16:00         20:00         0:00          4:00          8:00   12:00               16:00         20:00           0:00      4:00          8:00   12:00       16:00   20:00     0:00     4:00     8:00   12:00   16:00   20:00     0:00     4:00     8:00   12:00    16:00   20:00     0:00
Unit 1       ▽15:33 CWP(C) Manual stop
             ▲▽15:34 Emergency D/G A, B and H automatic start followed by immediate trip
             ▲15:36 MSIV manual full-close
              Water injection by RCIC (15:36 – 4:58)
                                                                                                                                                                                                                                                            ▲17:00 Reactor cold shutdown
             ▽15:57 CWP(A,B) Automatic stop
            ▲15:50 – as appropriate ▲Rapid depressurization (3:50 – 4:56)
            Reactor                     ▲0:00 MUWC alternative water injection start
            depressurization                          S/C cooling by FCS cooling water (MUWC) (6:20 – 7:20)
                                                   ▲D/W spray (7:10 – as appropriate)
                                                    ▲S/C spray (7:37 – as appropriate)
                                                                                                                                                                                             ▽9:40 MUWC alternative water injection stopped
                     ▲17:53 D/W cooling system                                                            Configuration of pressure proof vent-line (10:21 – 18:30)▲20:17 RHRS(B) Manual start
                     manual start                                                                                                                                                                  ▲21:03 RHRC(D) Manual start
                                                                                                                                                                                                           ▲1:24 RHR(B) Start  Call-off pursuant to Article 10 of the Act on Special Measures Concerning Special Law for Nuclear Emergency
                                                                                                                                                                                                             ▲3:39 S/C spray by RHR (B) start
                                                                                                                                                                                                                           ▲10:05 Reactor water injection by means of LPCI mode
                                                                                                                                                                                                                   ▲1:44 Emergency component cooling system (B) manual start
                                                                                                                                                                                                                                              ▲16:30 SFP water injection by FPMUW start                                          ▲0:42 RHR(B) SFP cooling start
                                                                                                                                                                                                                                                       ▲20:26 FPC(B) Circulation operation start
                                                                                                                                                                                                                                                                                                                                                        ▲10:30 SFP water temperature
                      Report pursuant to Article 10 of the Act on Special Measures Concerning Special Law for Nuclear Emergency (loss of reactor heat removal function) (18:33 on March 11 – 1:24 on March 14)
                                                                                                                                                                                                                                                                                                                                                        approximately 38°C (before
                                                           Report pursuant to Article 15 of the Act on Special Measures Concerning Special Law for Nuclear Emergency (loss of pressure suppression function) (5:22 on March 12 – 10:15 on March 14)                                                                                                     earthquake)

 [mm]                                                                                                                                                                                                                                                                                                                                                                            [MPa]
6000                                                                                                                                                                                                                                                                                                                                                                                   8.0
                                                                                                                                                                                                                                                                                                                                                                                       7.0
5000
                                                                                                                                                                                                                                                                                                                                                                                       6.0
4000                                                                                          Reactor water level [mm] (fuel area conversion)
                                                                                                                                                                                                                                                                                                                                                                                       5.0
3000                                                                                          Reactor pressure [MPa]                                                                                                                                                                                                                                                                   4.0
                                                                                                                                                                                                                                                                     Measurement missing
                                                                                                                                                                                                                                                                     due to paper jam                                                                                                  3.0
2000
                                                                                                                                                                                                                                                                                                                                                                                       2.0
1000
                                                                                                                                                                                                                                                                                                                                                                                       1.0
   0                                                                                                                                                                                                                                                                                                                                                                                   0.0
    12:00    16:00     20:00         0:00          4:00          8:00        12:00         16:00         20:00          0:00          4:00          8:00         12:00         16:00        20:00          0:00         4:00         8:00         12:00   16:00   20:00     0:00     4:00     8:00   12:00   16:00   20:00     0:00     4:00     8:00    12:00   16:00   20:00     0:00
[kPa]                                                                                                                                                                                                                                                                                                                                                                                  [°C]
 450                                                                                                                                                                                                                                                                                                                                                                                   160
 400                                                                                                                                                                                                                                                                                                                                                                                   140
 350                                                                                                                                                                                                                                                                                                                                                                                   120
 300
                                                                                                                                                                                                                                                                                                                                                                                       100
 250                                                                                                                                                                                                    D/W pressure [kPa abs]
                                                                                                                                                                                                                                                                                                                                                                                       80
 200                                                                                                                                                                                                    S/C pressure [kPa abs]
                                                                                                                                                                                                        S/C water temperature [°C]
                                                                                                                                                                                                                                                                                                                                                                                       60
 150
 100                                                                                                                                                                                                                                                                                                                                                                                   40

  50                                                                                                                                                                                                                                                                                                                                                                                   20
   0                                                                                                                                                                                                                                                                                                                                                                                   0
    12:00    16:00     20:00         0:00          4:00          8:00         12:00        16:00         20:00          0:00          4:00          8:00         12:00        16:00         20:00          0:00         4:00         8:00         12:00   16:00   20:00     0:00     4:00     8:00   12:00   16:00   20:00     0:00     4:00     8:00    12:00   16:00   20:00     0:00

                                                                                                                                   Chart 4-2: Situation of action in response to accident and plant parameter of Unit 1
                                                                                                                                                                                                                               25
March 11                                               March 12                                                                           March 13                                                                          March 14                                                    March 15                                             March 16
         12:00    16:00 20:00            0:00          4:00          8:00   12:00                16:00         20:00        0:00          4:00          8:00   12:00                16:00         20:00            0:00      4:00         8:00   12:00        16:00     20:00     0:00      4:00      8:00   12:00   16:00   20:00     0:00      4:00      8:00   12:00    16:00   20:00      0:00
Unit 2        ▲▽15:34 Emergency D/G H automatic start followed by immediate trip
              ▲15:34 MSIV manual full-close
              ▲15:35 RHR(B) Manual start
              ▽15:35 CWP(C) Manual stop, (A and B) Automatic stop
              ▽15:38 RHR(B) Stop
               ▲▽15:41 Emergency D/G A,B Automatic start followed by immediate trip                                                                                                                                                                                ▲18:00 Reactor cold shutdown
               Water injection by RCIC (15:41 – 4:53)
                 ▲15:41 – as appropriate Reactor depressurization as
                        ▲20:02 D/W cooling system manual start
                                                        ▲4:50 MUWC alternative water injection start
                                                            ▲D/W spray (7:11 – as appropriate)
                                                             ▲S/C spray (7:35 – as appropriate)                                                                                                                                       ▽10:12 MUWC alternative water injection stopped
                                                                     S/C cooling by FCS cooling water (MUWP) (6:20 – 7:52)                                                                                                ▲3:20 Emergency component cooling system (B) manual start
                                                                               Configuration of pressure proof vent-line (10:21 – 18:30)                                                                                   ▲3:51 RHRS(B) Manual start
                                                                                                                                                                                                                               ▲5:52 RHRC(B) Manual start
                                                                                                                                                                                                                                 ▲7:13 Start (S/C cooling mode)  Call-off pursuant to Article 10 of the Act on Special Measures Concerning Special Law for Nuclear Emergency
                                                                                                                                                                                                                                  ▲7:50 RHR(B) S/C spray start
                                                                                                                                                                                                                                        ▲10:48 RHR(B) Reactor water injection by means of LPCI mode
                                                                                                                                                                                                                                                                                                                                         ▲1:28 RHR(B) SFP cooling start
                       Report pursuant to Article 10 of the Act on Special Measures Concerning Special Law for Nuclear Emergencys (loss of reactor heat removal function) (18:33 on March 11 – 7:13 on March 14)
                                                                                                                                                                                                                                                                                                                                                               ▲
                                                             Report pursuant to Article 15 of the Act on Special Measures Concerning Special Law for Nuclear Emergency (loss of pressure suppression function) (5:32 on March 12 – 15:52 on March 14)                                                                                                          10:30 SFP water temperature
                                                                                                                                                                                                                                                                                                                                                               approximately 32.5°C (before
                                                                                                                                                                                                                                                                                                                                                               earthquake)
[mm]                                                                                                                                                                                                                                                                                                                                                                                       [MPa]
6000                                                                                                                                                                                                                                                                                                                                                                                           8.0
                                                                                                                                                                                                                                                                                                                                                                                               7.0
5000
                                                                                                                                                                                                                                                                                                                                                                                               6.0
4000                                                                                          Reactor water level [mm] (fuel area conversion)
                                                                                                                                                                                                                                                                                                                                                                                               5.0
3000                                                                                          Reactor pressure [MPa]                                                                                                                                                                                                                                                                           4.0
                                                                                                                                                                                                                                                                                                                                                                                               3.0
2000
                                                                                                                                                                                                                                                                                                                                                                                               2.0
1000
                                                                                                                                                                                                                                                                                                                                                                                               1.0
   0                                                                                                                                                                                                                                                                                                                                                                                           0.0
    12:00    16:00     20:00         0:00          4:00          8:00         12:00         16:00         20:00          0:00         4:00          8:00         12:00         16:00         20:00          0:00          4:00        8:00         12:00   16:00     20:00      0:00     4:00      8:00   12:00   16:00   20:00      0:00     4:00      8:00    12:00   16:00   20:00      0:00
                                                                                                                                                                                                                                                                                                                                                                                           [°C]
[kPa]
450                                                                                                                                                                                                                                                                                                                                                                                           160
400               ●:D/W圧力 [kPa abs]                                                                                                                                                                                                                                                                                                                                                           140
350               ●:S/C圧力 [kPa abs]                                                                                                                                                                                                                                                                                                                                                           120
300
                                                                                                                                                                                                                                                                                                                                                                                              100
250
                                                                                                                                                                                                    D/W pressure [kPa abs]                                                                                                                                                                    80
200
                                                                                                                                                                                                    S/C pressure [kPa abs]                                                                                                                                                                    60
150
                                                                                                                                                                                                    S/C water temperature [°C]
100                                                                                                                                                                                                                                                                                                                                                                                           40

 50                                                                                                                                                                                                                                                                                                                                                                                           20
  0                                                                                                                                                                                                                                                                                                                                                                                         0
   12:00     16:00    20:00         0:00          4:00          8:00         12:00         16:00         20:00         0:00          4:00          8:00         12:00         16:00         20:00         0:00            4:00       8:00         12:00    16:00    20:00    0:00        4:00      8:00   12:00   16:00   20:00   0:00        4:00      8:00   12:00    16:00   20:00   0:00

                                                                                                                                   Chart 4-3: Situation of action in response to accident and plant parameter of Unit 2
                                                                                                                                                                                                                                 26
March 11                              March 12                                                  March 13                                                March 14                                            March 15                                              March 16
         12:00     16:00 20:00     0:00     4:00     8:00   12:00       16:00     20:00      0:00     4:00     8:00   12:00     16:00   20:00     0:00     4:00        8:00   12:00    16:00   20:00     0:00     4:00     8:00   12:00    16:00     20:00     0:00     4:00     8:00   12:00    16:00   20:00      0:00
Unit 3       ▲15:37 MSIV manual full-close
             ▽15:38 CWP(B) Manual stop              ▲12:15 Reactor cold shutdown
             ▽15:34 CWP(C) Manual stop                                                                                                                                                                                                          ▲17:42 FPC Hx cooling water switch-over (RCW  RHRC)
             ▲▽15:35 Emergency D/G A, B and H automatic start followed by immediate trip of A
                                                                                                                                                                                                                                                                                                            ▲
             ▽15:33 CWP(C) Manual stop                                                                                                                                                                                                                                22:30 SFP water temperature approximately
             ▲▽15:34 Emergency D/G A, B and H automatic start followed by immediate trip of A and B                                                                                                                                                                                 32.5°C (before earthquake)
                  Water injection by RCIC (16:06 – 23:58)
                   ▽16:48 CWP(A) Manual stop

                                ▲22:53 MUWC alternative water injection start (unknown when stopped)
                                                               Configuration of pressure proof vent-line (12:08 – 12:13)                                                    ▲11:00 Emergency component cooling system (B) manual start
                 ▲15:36 RHR(B) S/C cooling start    ▲9:37 RHR(B) Manual start (SHC start)
                        ▲19:46 RHR(B) S/C cooling  switch-over to LPCI mode
                         ▲20:07 RHR(B) LPCI mode  switch-over to S/C cooling
                                 ▲0:06 RHR(B) Configuration of SHC mode system start
                                   ▽1:23 RHR(B) Manual stop (prepare for SHC mode being on)
                                      ▲2:39 RHR(B) Manual start (S/C cooling start)
                                      ▲2:41 RHR(B) S/C spray start
                                                 ▽7:59 RHR(B) Manual stop

                          ▲20:12 D/W cooling system manual start

  [mm]                                                                                                                                                                                                                                                                                                             [MPa]
6000                                                                                                                                                                                                                                                                                                                 8.0
                                                                                                                                                                                                                                                                                                                     7.0
5000
                                                                                                                                                                                                                                                                                                                     6.0
4000
                                                                                                                                                                                                                                                                                                                     5.0
                                                                    Reactor water level [mm] (fuel area conversion)
3000                                                                                                                                              Measurement missing                                                                                                                                                4.0
                                                                    Reactor pressure [MPa]                                                        due to paper jam                                                                                                                                                   3.0
2000
                                                                                                                                                                                                                                                                                                                     2.0
1000
                                                                                                                                                                                                                                                                                                                     1.0
   0                                                                                                                                                                                                                                                                                                                  0.0
    12:00    16:00      20:00    0:00     4:00     8:00     12:00    16:00    20:00       0:00      4:00     8:00     12:00   16:00   20:00     0:00     4:00        8:00   12:00   16:00   20:00      0:00     4:00     8:00   12:00   16:00     20:00      0:00     4:00     8:00   12:00   16:00   20:00       0:00

 [kPa]                                                                                                                                                                                                                                                                                                             [°C]
 450                                                                                                                                                                                                                                                                                                                 160
 400                                                                 D/W pressure [kPa abs]
                                                                                                                                                                                                                                                                                                                     140
                                                                     S/C pressure [kPa abs]
 350                                                                                                                                                                                                                                                                                                                 120
                                                                     S/C water temperature [°C]
 300
                                                                                                                                                                                                                                                                                                                     100
 250
                                                                                                                                                                                                                                                                                                                     80
 200
                                                                                                                                                                                                                                                                                                                     60
 150
 100                                                                                                                                                                                                                                                                                                                 40

  50                                                                                                                                                                                                                                                                                                                 20
   0                                                                                                                                                                                                                                                                                                              0
    12:00    16:00      20:00    0:00     4:00     8:00     12:00    16:00    20:00      0:00       4:00     8:00     12:00   16:00   20:00   0:00       4:00        8:00   12:00   16:00   20:00   0:00        4:00     8:00   12:00   16:00     20:00   0:00        4:00     8:00   12:00   16:00   20:00   0:00

                                                                                                  Chart 4-4: Situation of action in response to accident and plant parameter of Unit 3
                                                                                                                                                                27
March 11                                      March 12                                                              March 13                                                             March 14                                                         March 15                                                             March 15
         12:00    16:00 20:00        0:00        4:00       8:00   12:00           16:00       20:00      0:00        4:00        8:00   12:00           16:00      20:00       0:00       4:00        8:00   12:00           16:00      20:00      0:00     4:00       8:00   12:00          16:00      20:00      0:00       4:00          8:00   12:00        16:00      20:00    0:00
Unit 4       ▽15:33 CWP(C) Manual stop
             ▲▽15:34 Emergency D/G A, B and H automatic start followed by immediate trip of A
             ▽15:34
              d B CWP(A,B) Automatic stop         ▲11:17 HPCS pump start (S/C agitation operation)
             ▲15:36 MSIV manual full-close           ▲12:32 Reactor water injection switch-over (MUWC  HPCS)
             ▲15:36 RHR(A,B) Manual start               ▽13:48 Water injection by HPCS stopped
             ▽15:38 RHR(A) Manual stop
             ▽15:41 RHR(B) Automatic stop                                                                                                                                                                                                                            ▲7:15 Reactor cold shutdown
                 Water injection by RCIC (15:54 – 0:16)
                       ▲19:14 D/W cooling system manual start
                                   ▲0:16 MUWC alternative water         ▽17:25 MUWC alternative water injection stopped
                                   injection start ▲7:35 Alternative containment vessel spray start
                                                           S/C cooling by FCS cooling water start (7:23 – 22:14)                                                                                              ▲11:00 Emergency component cooling system (B) manual start
                                                                                                                                                                                                                    ▲13:07 RHRS(D) Manual start
                                                                        Configuration of pressure proof vent-line (11:44 – 11:52)                                                                                     ▲14:56 RHRC(B) Manual start
                                                                                                                                                                                                                        ▲15:42 RHR(B) Start (S/C cooling mode)  Call-off pursuant to Article 10 of the Act on Special Measures Concerning   Special Law for Nuclear Emergency
                                                                                                                                                                                                                         ▲16:02 RHR(B) S/C spray start
                                                                                                                                                                                                                             ▲18:58 RHR(B) Reactor water injection by means of LPCI mode start

                                                                                                                                                                                                                                                                                              ▲16:35 FPC Hx cooling water switch-over (RCW  RHRC)              ▲
                      Report pursuant to Article 10 of the Act on Special Measures Concerning Special Law for Nuclear Emergency (loss of reactor heat removal function) (18:33 on March 11 – 15:42 on March 14)
                                                                                                                                                                                                                                                                                                                                     20:59 RHR(B) SFP cooling start
                                                        Report pursuant to Article 15 of the Act on Special Measures Concerning Special Law for Nuclear Emergency (loss of pressure suppression function) (6:07 on March 12 – 7:15 on March 15)                                                                         (SFP water temperature approximately 35°C
                                                                                                                                                                                                                                                                                                                                               (before earthquake))

 [mm]                                                                                                                                                                                                                                                                                                                                                                               [MPa]
6000                                                                                                                                                                                                                                                                                                                                                                                  8.0
                                                                                                                                                                                                                                                                                                                                                                                      7.0
5000
                                                                                                                                                                                                                                                                                                                                                                                      6.0
4000
                                                                                                                                                                                                                                                                                                                                                                                      5.0
                                                                                Reactor water level [mm] (fuel area conversion)
3000                                                                                                                                                                                                                                                                                                                                                                                  4.0
                                                                                Reactor pressure [MPa]                                                                                                                                                                                                                                                                                3.0
2000
                                                                                                                                                                                                                                                                                                                                                                                      2.0
1000
                                                                                                                                                                                                                                                                                                                                                                                      1.0
   0                                                                                                                                                                                                                                                                                                                                                                                 0.0
    12:00    16:00    20:00       0:00       4:00        8:00       12:00      16:00       20:00       0:00        4:00       8:00       12:00      16:00       20:00       0:00        4:00        8:00      12:00       16:00      20:00        0:00     4:00      8:00      12:00      16:00      20:00       0:00       4:00      8:00         12:00     16:00      20:00    0:00

 [kPa]                                                                                                                                                                                                                                                                                                                                                                               [°C]
 450                                                                                                                                                                                                                                                                                                                                                                                 160
 400                                                                                                                                                                                                                                                                                                                                                                                 140
 350                                                                                                                                                                                                                                                                                                                                                                                 120
 300
                                                                                                                                                                                                                                                                                                                                                                                     100
 250
                                                                                                                                                                                                                                                                                                                                                                                     80
 200
                                                                                                                                                                                                            D/W pressure [kPa abs]                                                                                                                                                   60
 150                                                                                                                                                                                                        S/C pressure [kPa abs]
 100                                                                                                                                                                                                        S/C water temperature [°C]
                                                                                                                                                                                                                                                                                                                                                                                     40

  50                                                                                                                                                                                                                                                                                                                                                                                 20
   0                                                                                                                                                                                                                                                                                                                                                                                 0
    12:00    16:00    20:00      0:00        4:00       8:00       12:00       16:00      20:00       0:00        4:00        8:00      12:00       16:00      20:00        0:00       4:00         8:00      12:00      16:00       20:00        0:00     4:00      8:00      12:00      16:00      20:00      0:00       4:00       8:00        12:00      16:00      20:00    0:00

                                                                                                                Chart 4-5: Situation of action in response to accident and plant parameter of Unit 4
                                                                                                                                                                                               28
4.1 Response status from the time of the earthquake and tsunami until restoration
    and cold shutdown
Just before the earthquake, all Units 1 through 4 were in operation at the rated power. All
control rods were fully inserted by the automatic scram signal triggered by the earthquake,
and all reactors shut down automatically. In this way, the water level in the reactor was
lowered to "(L-3) low reactor water level," but the normal level was recovered without
reaching the level which would automatically activate the emergency core cooling system
by supplying water from the reactor feedwater system. Further, as designed, the "(L-3)
reactor water level low" automatically activated the containment vessel isolation system
and the emergency gas treatment system, and isolation of the containment vessel and
negative pressure of the reactor building were achieved.
After the entry of the tsunami, the main condenser could not be used as a heat sink, so the
main steam isolation valve was fully closed manually (core isolation) and reactor pressure
was controlled by the main steam safety relief valve. In this operation, in accordance with
the core isolation operating procedures (when the main steam isolation valve is closed), the
reactor core isolation cooling system (hereinafter referred to as "RCIC") was manually
started up in order to control the water level of the reactor by repeating automatic shutdown
and manual start up based on the water level.
The entry of the tsunami caused part of the emergency core cooling system to become
unusable due to the flooding of the pump motor and power source in the seawater system.
Water was injected to the core by RCIC and the main steam safety relief valve was opened
to depressurize the reactor core in order to maintain the alternate water injection
(application of Accident Management (AM) procedure) via the condensate water makeup
system (hereinafter referred to as "MUWC"). Before switching over to MUWC, it was
confirmed that water injection via MUWC to the reactor core would be possible with RCIC
in operation. The water temperature of the S/C rose. Therefore, water was injected to the
S/C using the drain line in the water cooler of the combustible gas control system to cool
down the containment vessel, together with dry well spray and S/C spray. This dry well
spray was executed at the discretion of the Director at the power plant.
The equipment at the south area of the heat exchanger building of Unit 3 was not flooded,
and so the emergency D/G and the residual heat removal system remained operative.
Therefore, Unit 3 was able to achieve cold shutdown using the cooling system that is used
in normal reactor shutdown cooling mode.
An external power supply was available through Tomioka 1L and a number of operations,
and monitoring of major parameters were possible at the central control room with little
damage on transformers and power panel, making the emergency responses quick and
resulting in early convergence. Expeditious recovery was tried on the damaged line before
it was lost; Iwaido 2L recovered at 1:38 pm on the 12th, and Iwaido 1L recovered at 5:15
am on the 13th. It was fortunate that the plant was able to use Tomioka IL continuously from
the very beginning without power outage. This situation made it possible to respond to the
accident appropriately and prevented the expansion of the accident.
The development of AM measures such as the use of MUWC contributed significantly to
the convergence of the accident at Fukushima Daini. Also, as another AM measure which
would have been quickly available if the accident situation had rapidly deteriorated, a
containment vent facility was prepared and available, although it was ultimately not used
because of the swift convergence of the accident. When a report was sent to the national
and local governments in accordance with Article 10 of the Nuclear Disaster Special
Measures Law (hereinafter referred to as "Nuclear Emergency Law"), TEPCO explained
that they were preparing to implement a vent from the containment vessel.
In parallel with the response operations described above, the plant’s emergency
headquarters (hereinafter referred to as "Headquarters") made a thorough check of the

                                           29
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